The moment Microsoft announced it would lock other browsers out of being installed on Windows RT, we all knew regulatory bodies the world over were wringing their hands. Today, this has been confirmed: in the wake of an investigation into Microsoft not complying with the existing antitrust rulings regarding browser choice, the EU has also announced it's investigating Windows 8 x86 and Windows 8 RT (ARM).

iOS mandates that 3rd-party browsers use the webkit engine that comes with iOS, though. So Firefox is banned,because Firefox uses gecko. Only if Mozilla changed Firefox to use the iOS webkit engine, would Apple allow Firefox to run on iOS.

Microsoft isn't nearly as harsh in that respect. Microsoft will allow Firefox to run on WindowsRT, but Mozilla says that their javascript engine won't run fast because WindowsRT lacks access to certain Win32 apis (WinRT only supports a subset of Win32 api).
So Microsoft is already treating Mozilla better than Apple does. Moreover, just as Mozilla could change Firefox to use webkit when running on iOS, Mozilla could change Firefox to use the IE engine (I think it's called "Trident"), when running on WindowsRT.

So WindowsRT is at least as permissive as iOS wrt Mozilla, and actually more so. Mozilla is being hypocritical by complaining about WindowsRT (which as zero percent marketshare, BTW) while not lodging similar complaints against iOS, which has a harsher policy AND controls 80% of the ARM market.